On Thu, 5 Jan 2012 09:48:37 -0200
Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote:
> On Wed, Jan 04, 2012 at 03:06:43PM +0900, Takuya Yoshikawa wrote:
> > It is possible that the __set_bit() in mark_page_dirty() is called
> > simultaneously on the same region of memory, which may result in only
> > one bit being set, because some callers do not take mmu_lock before
> > mark_page_dirty().
> >
> > This problem is hard to produce because when we reach mark_page_dirty()
> > beginning from, e.g., tdp_page_fault(), mmu_lock is being held during
> > __direct_map(): making kvm-unit-tests' dirty log api test write to two
> > pages concurrently was not useful for this reason.
> >
> > So we have confirmed that there can actually be race condition by
> > checking if some callers really reach there without holding mmu_lock
> > using spin_is_locked(): probably they were from kvm_write_guest_page().
> >
> > To fix this race, this patch changes the bit operation to the atomic
> > version: note that nr_dirty_pages also suffers from the race but we do
> > not need exactly correct numbers for now.
> >
> > Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx>
> > ---
> > virt/kvm/kvm_main.c | 2 +-
> > 1 files changed, 1 insertions(+), 1 deletions(-)
> >
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 7287bf5..a91f980 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -1543,7 +1543,7 @@ void mark_page_dirty_in_slot(struct kvm *kvm, struct kvm_memory_slot *memslot,
> > if (memslot && memslot->dirty_bitmap) {
> > unsigned long rel_gfn = gfn - memslot->base_gfn;
> >
> > - if (!__test_and_set_bit_le(rel_gfn, memslot->dirty_bitmap))
> > + if (!test_and_set_bit_le(rel_gfn, memslot->dirty_bitmap))
> > memslot->nr_dirty_pages++;
> > }
> > }
>
> Looks good to me.
>
I was planning to avoid bitmap flip based on srcu by clearing each bit
right before rmap write protection.
for each gfn_offset
__clear_bit();
nr_dirty_pages--;
rmap_write_protect();
This way, I could eliminate memslots allocations and srcu synchronizations
and get dirty log became faster.
Furthermore get dirty log could be called for selected pages without
affecting others.
Now I need to re-think how to achieve the same thing, and measure the effect
again.
Takuya
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